Pertussis Cocoon Strategy for Latin America
Among vaccine-preventable diseases and deaths in children, pertussis is the most common cause in those countries where pediatric universal pneumococcal conjugate vaccination has been introduced in national immunization programs. Despite good vaccination coverage rates of the primary series and booster doses in children from many countries, outbreaks and epidemics still occur. Owing to waning immunity over the years among adults and adolescents, these individuals including new parents are the most common source of transmission to young infants and, therefore, have become important target groups for selective immunization strategies. Despite the availability of new acellular pertussis vaccines (Tdap) for these groups and accumulative experience with their use during the last decade, the disease burden remains considerable in many countries.

The pertussis cocoon strategy consists of vaccinating with Tdap: women, their partners and other close contacts of young infants. It has been recommended by the Advisory Committee on Immunization Practices in the USA and many other health authorities in different countries as a potential useful strategy to decrease the impact of the disease. If perfectly implemented, cocoon strategy has been estimated to reduce pertussis cases in young infants by 70%. In 2008, specific immunization recommendations for pregnant and postpartum women were published by the Advisory Committee on Immunization Practices and since then updated recommendations have been released. Whenever possible, it is preferred that women get vaccinated with Tdap before getting pregnant. If not, postpartum vaccination of new mothers and their couples has been recommended until more recently, when Tdap during pregnancy is recommended, preferably during the third or late second trimester (after 20 weeks of gestation). Compared with postpartum Tdap vaccination, its use during pregnancy has been found to be more cost effective and prevents greater number of infant cases and fatalities. However, if not given to pregnant women, postpartum Tdap should be applied as soon as possible to women, as immediate antibody response to the vaccine in them and breast milk antibodies are not achieved early in the first week of life. In an interesting recent kinetics study measuring serum and breast milk antibodies to the different components of Tdap in childbearing and postpartum women, Halperin et al. showed how the increase to reach peak antibody levels requires at least 1–2 weeks, and therefore the infant would not be protected in the first 2 weeks of life. Gall and colleagues demonstrated the potential benefit of giving Tdap during pregnancy in terms of high titers of pertussis antibodies to the vaccine antigens in the serum of pregnant vaccinated mothers and their newborns.

Some developed and developing countries have adopted the recommendations of cocooning with Tdap vaccine. The success of this strategy has not yet been well established and its impact will depend on many factors. For example, for countries where the rates of pregnancies without prenatal care and extrahospitalary deliveries are considerable, the impact of this strategy would be minimal, especially in countries with low coverage rates of primary TDP or TDaP series, and no adolescent vaccination. On the other hand, it is important to consider that a country may adopt different strategies depending on the number of maternities, geographical extension and the logistics, among other factors; however, the most important issue is to reach good vaccination coverage rates with the selected strategy. Vaccinating only postpartum or pregnant women and their couples will not resolve pertussis outbreaks and epidemics, and a combination of different strategies seem to be the best option for controlling pertussis. Being realistic, among vaccine preventable diseases, for many reasons pertussis will be very difficult to eradicate worldwide at least for many decades. Theoretically, the best way to decrease significantly the number of pertussis cases and deaths worldwide would be having acceptable primary series coverage rates, universal adolescent and adult vaccination, and cocoon strategy implementation. However, economical, logistical, acceptability and other aspects of these interventions would make these strategies very difficult to be implemented widely.

The first country to implement a nation-wide maternal postpartum Tdap vaccination was Costa Rica in April of 2007, owing to a severe outbreak. It was expected at that time that without adolescent and adult universal vaccination in the country, the impact of this outbreak would have been considerable. Therefore, the main goal of this strategy was to reduce predominantly the associated high fatality rate in the specific group of infants younger than 2 months. In this country, the vaccine has been offered at no extra cost since then to all mothers and their partners who have delivered their babies in all maternities of the Social Security System. The impact of the intervention is under investigation; however, mortality rates attributed to pertussis have decreased since then. Among other developing countries, Panama also introduced maternal postpartum Tdap vaccine owing to a severe outbreak and has recently moved to pregnancy vaccination. In February 2012, Argentina started universal Tdap vaccination during pregnancy owing to recent outbreaks and more than 70 infant fatalities in year 2011. Among Latin American countries recently experiencing outbreaks, Uruguay, Chile, Mexico, Brazil and Colombia, have introduced or will introduce Tdap vaccination among different selected high-risk populations such as postpartum and pregnant women, adolescents and healthcare workers. Information from other developing countries using Tdap cocoon strategy is limited.

Among developed countries, Canada, the USA, Australia and some European countries have introduced postpartum or maternal Tdap vaccination in selected hospital, maternities and territories. Strategies have also included parental or close family contacts vaccination at pediatric offices, workplaces, neonatal units, hospitals and other health care settings. Among these vaccine candidates, parents and particularly mothers have the highest vaccine acceptance rates. In Houston, a renamed pertussis experts group recently reported their experience with postpartum Tdap given prior to hospital discharge in 5223 women (67% of postpartum women) from January 2008 to May 2009. Although this was a leading and very important study in terms of postpartum Tdap implementation strategies, the authors failed to demonstrate the impact in terms of pertussis incidence by vaccinating only postpartum mothers with Tdap. Among several limitations that the authors acknowledge in their work and in an accompanying editorial, the number of vaccinated women and pertussis cases was small. A recent study addressed the potential impact of parental Tdap immunization on pertussis hospitalizations in US infants. The authors found that the reduction of cases would be greater if both parents are vaccinated compared with vaccination only in the mother. Also, the reduction would be greater if parents are immunized at least 2 weeks prior to delivery when compared with when the vaccine is administered at delivery or at the 2-week newborn visit.

During pertussis outbreaks, accelerating the primary series vaccination schedule of infants has been suggested as a potential strategy to decrease pertussis infant cases, hospitalization rates and fatalities. Models have estimated the impact of this intervention and the number of individuals that would be needed to vaccinate to impact positively those three scenarios. However, the number of parents that would need to be vaccinated through Tdap cocoon strategy to prevent a neonatal or infant pertussis hospitalization or death was unknown until recently, when Canadian investigators published the first study to address this situation. This study was performed taking into account the experience and information of two of the largest provinces in Canada and where outbreaks have occurred in the past, British Columbia and Quebec. They found that the number needed to vaccinate for parental immunization was at least 1,000,000 to prevent one infant fatality, 100,000 for admission to an ICU and more than 10,000 for a hospitalization. In this study, considering the low incidence rates of pertussis in the two provinces they analyzed during the study period, parental cocooning may be inefficient and resource intensive. The study has several limitations and the authors acknowledge this. They conclude that these models should be investigated in other countries where pertussis incidence, number of hospital admissions and fatalities, and socioeconomic costs attributed to the medical attention of these patients and their close contacts may be higher.

In conclusion, although cocooning with Tdap either in pregnant or postpartum women may be difficult to implement in some developing and developed countries, it should be recommended as a useful complementary strategy to fight against this disease. Evidence from countries and sites that have implemented this strategy should be diffused to the medical and public health community. Its applicability and impact should be considered in the light of countries' priorities in terms of pertussis incidence and disease burden, fatality rates, outbreaks, coverage rates and introduction of other vaccines, among others. All efforts to achieve higher vaccine coverage rates among different age groups and populations, will contribute to decrease the impact of this old vaccine-preventable disease. Even in the era of advanced technology, modern vaccines and improvements in the management of the critically ill infant or newborn with pertussis, outbreaks and epidemics still occur even in developed countries. The reasons for these are multifactorial, and outbreaks like the recent ones in the USA, Latin America, Europe, Australia and Asia, make pertussis one of the oldest but most difficult to control vaccine-preventable disease.